在Microsoft.Extensions.DependencyInjection中，Singleton代表單例依賴注入，但是這個單例也只是對於相同的ServiceProvider物件而言。

假設我們的專案中有介面IPeople和實現類People，它們之間的依賴注入關係是Singleton的。

新建一個.NET Core控制檯專案，程式碼如下所示：

using Microsoft.Extensions.DependencyInjection;
using System;

namespace NetCoreDISingletonDemos
{
    interface IPeople
    {

    }

    
 
class People : IPeople
    {

    }

    class Program
    {
        static void Main(string[] args)
        {
            IServiceCollection services = new ServiceCollection();
            services.AddSingleton<IPeople, People>();//註冊介面IPeople和類People的關係為Singleton

            IPeople peopleA = null
 
;
            IPeople peopleB = null;

            ServiceProvider serviceProvider = null;

            //下面測試在同一個ServiceProvider中獲取IPeople物件兩次，賦值給peopleA和peopleB變數
            using (serviceProvider = services.BuildServiceProvider())
            {
                peopleA = serviceProvider.GetService<IPeople>();
                peopleB 
 
= serviceProvider.GetService<IPeople>();

                Console.WriteLine("peopleA==peopleB?{0}", (peopleA == peopleB).ToString());//True
            }

            peopleA = null;
            peopleB = null;

            //下面測試在不同的ServiceProvider中獲取IPeople物件兩次，賦值給peopleA和peopleB變數
            using (serviceProvider = services.BuildServiceProvider())
            {
                peopleA = serviceProvider.GetService<IPeople>();
            }

            using (serviceProvider = services.BuildServiceProvider())
            {
                peopleB = serviceProvider.GetService<IPeople>();
            }

            Console.WriteLine("peopleA==peopleB?{0}", (peopleA == peopleB).ToString());//False

            peopleA = null;
            peopleB = null;

            //下面測試用同一個ServiceProvider建立的不同IServiceScope，來獲取IPeople物件兩次，賦值給peopleA和peopleB變數
            using (serviceProvider = services.BuildServiceProvider())
            {
                IServiceScope serviceScopeA = null;
                IServiceScope serviceScopeB = null;

                using (serviceScopeA = serviceProvider.CreateScope())
                {
                    peopleA = serviceScopeA.ServiceProvider.GetService<IPeople>();
                }

                using (serviceScopeB = serviceProvider.CreateScope())
                {
                    peopleB = serviceScopeB.ServiceProvider.GetService<IPeople>();
                }

                Console.WriteLine("serviceScopeA.ServiceProvider==serviceScopeB.ServiceProvider?{0}", (serviceScopeA.ServiceProvider == serviceScopeB.ServiceProvider).ToString());//False
                Console.WriteLine("peopleA==peopleB?{0}", (peopleA == peopleB).ToString());//True
            }

            Console.WriteLine("Press any key to end...");
            Console.ReadKey();
        }
    }
}

執行結果如下所示：

可以看到：

• 我們使用同一個ServiceProvider獲取到的兩個IPeople物件是相同的。
• 我們使用不同的ServiceProvider獲取到的兩個IPeople物件是不同的。
• 我們使用同一個ServiceProvider建立的不同IServiceScope，獲取到的兩個IPeople物件是相同的。

關於Microsoft.Extensions.DependencyInjection的Singleton依賴注入關係，可以參考這裡。